require selfがよくわからなかったけどここに書いてあった

Sprockets uses manifest files to determine which assets to include and serve. These manifest files contain directives — instructions that tell Sprockets which files to require in order to build a single CSS or JavaScript file. With these directives, Sprockets loads the files specified, processes them if necessary, concatenates them into one single file and then compresses them (if Rails.application.config.assets.compress is true). By serving one file rather than many, the load time of pages can be greatly reduced because the browser makes fewer requests. Compression also reduces the file size enabling the browser to download it faster.